1. Field of the Invention
The invention relates to integrated circuit packaging schemes, and more particularly to a scheme by which a die fabricated according to a single layout can be packaged in either of two or more different package types.
2. Description of Related Art
When an integrated circuit die is to be packaged in an integrated circuit package, the die is typically placed in the center of a lead frame consisting of a plurality of leads extending radially from the position where the die is placed. The inside ends of these radially extending leads are referred to for historical reasons as bonding posts, even though they may not actually look like posts once the package is completed. Extremely thin wires are then typically bonded between each of the posts and corresponding bonding pads which are integrated along the four sides of the die.
If the package is to be plastic, the die/lead frame assembly is then placed in a mold and hot plastic is injected in. Since the hot plastic is fairly viscous, it tends to sweep the wires somewhat in the direction in which the plastic is traveling. If the wires get too close to each other, they will short. Thus it is highly undesirable to run a bonding wire from a pad on one side of the die to a bonding post on a different side of the die. For example, it is usually considered too risky to run a wire from one side of the die to the side of posts facing a different but adjacent side of the die, even if the pad is right next to the corner pad between the two sides. If the package is to be ceramic or cerdip, the top and bottom halves of the package are placed around the die/lead frame assembly and fixed into place. The risk of shorting wires is not as great in ceramic packages as it is in plastic packages, but it is nevertheless still considered undesirable to run a wire from a pad on one side of the die to a post on any other side of the die.
Recent years have seen a proliferation of different package types, including dual in-line packages (DIPs), quad in-line packages (QIPs), quad flat packs (QFPs), plastic leaded chip carriers (PLCCs) and pin grid arrays (PGAs). All of these packages are available with different standard numbers of pins on a side, and consequently, different numbers of bonding posts on each side of the die. Some packages, such as TO-type packages, even have their bonding posts arranged in a circle around the die. Moveover, to further complicate the bonding process, many new package and die designs permit very large numbers of connections between the die and the outside world. Most of the package types are available only in standard sizes with fixed pin counts.
For various important marketing and/or applications reasons, a manufacturer may desire to produce a particular integrated circuit design in particular ones of these packages. Often it is desirable to produce the circuit in several types of packages, in order to give the customer a choice. Thus, for example, the manufacturer may choose to offer a particular integrated circuit both in an 80-pin QFP and an 84-pin PLCC. (PLCC's are generally not available with 80 pins.) The lead frames for both these packages are arranged to have bonding posts on all four sides of the die, but the number of bonding posts located on one side of the die in one package is different than the number of bonding posts located on the same side of the die in the other package. For the 80-pin QFP, the posts are arranged in a 16.times.24 post rectangle, whereas in the 84-pin PLCC, they are arranged in a 21.times.21 post rectangle.
If the integrated circuit requires, for example, 80 connections to the outside world, then the same die design cannot be used in both packages. If the die is to be packaged in the 80-pin QFP, then the pads must be integrated thereon in a 16.times.24 pad format. Any other arrangement would require a wire to be bonded from a pad on one side of the die to a post on a different side of the die. But if the same die is packaged in the 84-pin PLCC, some of the bonding posts located on a 16-pad side of the die would have no immediately facing pad, and would have to be wire bonded with a pad located on the 24-pad side of the die. Thus it can be seen that the same die usually cannot be packaged in more than one different package type.
In the past, the typical solution to the above problem would have been to manufacture dies with a different layout for each desired package type. This cannot be accomplished easily. Merely moving pads from one side of the die to an adjacent side is impractical because usually there is not enough room on the destination side to accommodate more pads. Instead, a significant circuit re-layout task usually must be done to make the die fit each package in which the circuit is to be offered.
The problem of having to manufacture a different die type for each desired package type has a number of important consequences. First, merely generating a new mask set is a very expensive proposition. Second, the development work required to generate a new layout is expensive and time consuming. Thus, either the marketing decision as to which package tYpe will be used must be made long before the product will actually be available to the market, or a duplicate design effort must be undertaken. Third, a new layout must be debugged entirely independently of the old, must be independently checked for design rule compliance, and risks having electrical properties which differ from those of the old layout. Fourth, the manufacture of a second die type requires additional processing, tooling, handling and inventory than would be required for a single die type.